Decanethiol Monolayers Research Articles

Low coverage disordered decanethiol monolayers on Au(001): A conjecture regarding the formation of Au-adatom-molecule complexes

We present a scanning tunneling microscopy study of decanethiol on Au(001) in the low coverage regime. As expected, the hex reconstruction is lifted, however, no ordered decanethiol phases form. We observe large areas free of Au islands and covered with a disordered decanethiol phase. I(t) spectroscopy measurements suggest that this disordered phase is dynamic and most likely comprises diffusing Au adatoms, decanethiol molecules, and/or Au-adatom-decanethiol molecule complexes. We have performed density functional theory calculations and show that the activation barrier for diffusion is lower when Au-adatom-molecule complexes are considered in comparison to the case of bare molecule. These findings suggest that although no vacancy pits form on Au(001), Au-adatoms expelled during the lifting of the hex reconstruction may be still important for the diffusion of thiol molecules on this surface.

An atom probe analysis of self-assembled monolayers: A novel approach to investigate mixed and unmixed self-assembled monolayers (SAMs) on gold

Atom probe tomography was used to analyze self-assembled monolayer of octanethiol and (1H,1H,2H,2H) perfluoro decanethiol (PFDT) in a mixed and unmixed state on a gold surface. The first part of this report, we have analyzed the two SAMs with a direct comparison, whereas in the second part, a binary mixture of these two SAMs (with a 1:1 ratio) are analyzed. Au was taken as a metal substrate in both cases. It has been observed that, although having similar structure and belongs from the same SAMs group, the sequence of evaporation of both chains appeared significantly different molecular fractions in the first case, while in the mixed phase condition the two SAMs chains were arranged in a patchy domain by forming a small nanoscale cluster on the gold surface. Primarily, this study gives an overall understanding of evaporation sequence and surface morphology of the SAMs in an unmixed and mixed condition on the gold substrate, which may lead to the further design process in various applications.

Correction to "Molecular Dynamics and Energy Landscape of Decanethiolates in Self-Assembled Monolayers on Au(111) Studied by Scanning Tunneling Microscopy".

The energetics and dynamics of the various phases of decanethiolate self-assembled monolayers on Au(111) surfaces were studied with scanning tunneling microscopy. We have observed five different phases of the decanethiolate monolayer on Au(111): four ordered phases (β, δ, χ*, and ) and one disordered phase (e). We have determined the boundary free energies between the disordered and order phases by analyzing the thermally induced meandering of the domain boundaries. On the basis of these results, we are able to accurately predict the two-dimensional phase diagram of the decanethiolate/Au(111) system. The order–disorder phase transition of the χ* phase occurs at 295 K, followed by the order–disorder phase transition of the β phase at 325 K. Above temperatures of 325 K, only the densely packed and disordered e phases remain. Our findings are in good agreement with the phase diagram of the decanethiolate/Au(111) system that was put forward by Poirier et al

Ordering of Air-Oxidized Decanethiols on Au(111).

Self-assembled monolayers (SAMs) of alkanethiols on gold are a commonly used platform for nanotechnology owing to their ease of preparation and high surface coverage. Unfortunately, the gold–sulfur bond is oxidized at ambient conditions which alters the stability and structure of the monolayer. We show using scanning tunneling microscopy and X-ray photoelectron spectroscopy that decanethiolate molecules oxidize into decanesulfonates that organize into a hitherto unknown striped phase. Air-exposed SAMs oxidize, as can be determined by a shift of the S 2p peak and the appearance of O 1s photoelectrons as part of the decanethiol monolayer transforms into a lamellae-like decanesulfonate structure when exposed to air. The herringbone structure of the Au(111) surface is preserved, indicating that the interaction between the molecules and the surface is rather weak as these findings are substantiated by density functional theory calculations.

In Situ Vibrational Study of the Reductive Desorption of Alkanethiol Monolayers on Gold by Sum Frequency Generation Spectroscopy

In situ sum frequency generation vibrational spectroscopy (SFG) was used to monitor the reductive desorption of decanethiol (DT) and octadecanethiol (ODT) self-assembled monolayers (SAMs) grown on evaporated gold on silica. At negative potentials, the alkyl chains of both monolayers became disordered as monitored by the appearance of methylene symmetric stretching modes in the collected spectra. The increased tilting of the terminal methyl groups on the chains of the DT monolayer further support this observation. The disappearance of the methyl C–H stretching vibrational modes at the reductive potential suggests that DT molecules diffused away from the surface after reduction. ODT molecules, on the other hand, retained their two-dimensional structure near the gold surface, implied by the strong methyl vibrational modes at the reductive potential. After four reductive cycles, a large portion of the DT SAM was fully desorbed, while the ODT monolayer existed as a combination of physisorbed and chemisorbed mo...

Non-seeded synthesis and characterization of superparamagnetic iron oxide nanoparticles incorporated into silica nanoparticles via ultrasound

A non-seeded method of incorporating superparamagnetic iron oxide nanoparticles (SPION) into silica nanoparticles is presented. Mixture of both SPION and silica nanoparticles was ultrasonically irradiated. The collapsed bubbles and shockwave generated from the ultrasonic irradiation produce tremendous force that caused inelastic collision and incorporation of SPION into the silica. Physicochemical analyses using transmission electron microscope (TEM), electronic spectroscopic imaging (ESI), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy demonstrated the formation of SPION/silica composite nanoparticles. The prepared composite nanoparticles exhibited superparamagnetic behaviour and nearly 70% of the initial saturation magnetization (Ms) of the SPION was retained. The presence and reactivity of the silica were demonstrated via assembling decanethiol monolayer on the composite nanoparticles. The silanol group of the silica provided the binding site for the alkyl group in the decanethiol molecules. Therefore, the thiol moiety became the terminal and functional group on the magnetic composite nanoparticles.

An <i>In-Situ</i> Functionalization of Decanethiol Monolayer on Thin Silica Coated Superparamagnetic Iron Oxide Nanoparticles Synthesized by Non-Seeded Process

Superparamagnetic iron oxide nanoparticles (SPION) are routinely employed in biomedical and water treatment applications due to their inherent magnetic properties. However, for these applications a surface modified and functionalized SPION is required. Herein, for the first time we present an in-situ method of functionalizing decanethiol monolayer on thin silica coated SPION synthesised via a non-seeded process. Through physico-chemical analyses the grafting of decanethiol onto the composite nanoparticles were demonstrated. The alkane functional group in the monolayer molecule attached to the silanol group of the silica shell and became the tail and binding site to the magnetic composite nanoparticles.

Molecular Dynamics and Energy Landscape of Decanethiolates in Self-Assembled Monolayers on Au(111) Studied by Scanning Tunneling Microscopy

The energetics and dynamics of the various phases of decanethiolate self-assembled monolayers on Au(111) surfaces were studied with scanning tunneling microscopy. We have observed five different phases of the decanethiolate monolayer on Au(111): four ordered phases (β, δ, χ*, and φ) and one disordered phase (ε). We have determined the boundary free energies between the disordered and order phases by analyzing the thermally induced meandering of the domain boundaries. On the basis of these results, we are able to accurately predict the two-dimensional phase diagram of the decanethiolate/Au(111) system. The order-disorder phase transition of the χ* phase occurs at 295 K, followed by the order-disorder phase transition of the β phase at 325 K. Above temperatures of 325 K, only the densely packed φ and disordered ε phases remain. Our findings are in good agreement with the phase diagram of the decanethiolate/Au(111) system that was put forward by Poirier et al. [Langmuir 2001, 17 (4), 1176-1183].

A new striped-phase of decanethiol self-assembled monolayers on Au(1 1 1) formed at a high solution temperature

Self-assembled monolayers (SAMs) of decanethiol (C10) on Au(111) prepared at room temperature (RT), 60°C, and 70°C have been investigated using high resolution scanning tunneling microscopy (STM). For SAMs prepared at RT, the c(4×2) superlattice of the basic (3×3)R30° structure was hardly observed in small-scale STM images. Increasing the solution temperature to 60°C results in formation of SAMs having large ordered domains of the c(4×2) superlattice. In contrast to the previous studies, our results indicate the presence of four different topographical heights for the C10 molecules per c(4×2) unit cell. Preparation of C10 SAMs from 70°C of solution temperature leads to the formation of new striped phase with (43×23)R30° unit cell. The area of a single molecule in the (43×23)R30° striped structure amounts to 57.73Å2, which is about 167% larger than that (21.6Å2) for a molecule in the c(4×2) structure.

Surface-Enhanced Vibrational Spectroscopy of Adsorbates on Microemulsion Synthesized Gold Nanoparticles

Very small (<10 nm) monodisperse gold nanoparticles (AuNPs) coated with a monolayer of decanethiol were prepared and their surface-enhanced infrared absorption (SEIRA) spectra were measured in the transmission mode. The AuNPs were prepared by the borohydride reduction of HAuCl(4) inside reverse micelles that were made by adding water to a hexane solution of sodium bis(2-ethylhexyl)sulfosuccinate (AOT). The gold nanoparticles were then stabilized by the addition of decanethiol. Subsequent addition of p-nitrothiophenol both facilitated the removal of excess AOT and showed that the gold surface was completely covered by the decanethiol. SEIRA spectra of decanethiol on gold particles prepared in AOT microemulsions were about twelve times more intense than corresponding layers on gold produced by electroless deposition and gave a significantly less noisy spectrum compared to the corresponding surface-enhanced Raman spectrum. The surface-enhanced Raman scattering (SERS) spectra of the same samples showed that the most intense spectrum was obtained from gold nanoparticles with a mean diameter of 2.5 nm. This result is in contrast to previous statements that SERS spectra could only be obtained from particles larger than 10 nm.

Deposition of Laminar TiO&lt;SUB&gt;2&lt;/SUB&gt;-Based Nanocomposites on a Modified Quartz Crystal Gold Surface

The deposition of hybrid organic-inorganic nanocomposites of titanium dioxide--prepared by hydrolysis of titanium tretraisopropoxide modified through exchanging one of its ligands with a butinoxy-, octiloxy- or phenoxy-group--on a gold surface modified by a monolayer of decanethiol was studied using a Quartz Crystal Microbalance. The depositions, performed from a suspension in alcohol of the nanocomposites, consisting in titanium oxide bilayers hydrophobically functionalized, occurs by a spontaneous process 'layer by layer.' However the amount of deposited mass in a layer as well as the rates of deposition depend on the degree of aggregation of the nanocomposite in the suspension which is in turn determined by nature of the organic ligands.

Effects of concentration and temperature on the formation process of decanethiol self-assembled monolayer on Au(1 1 1) followed by electrochemical reductive desorption

The formation process of self-assembled monolayer (SAM) of decanethiol (C10SH) on Au(1 1 1) single crystal electrode has been investigated for wide range of C10SH concentration (0.1–500 μM) and temperature (253–298 K). The amount and quality of C10SH SAM were determined from area and position, respectively, of reductive desorption peak of the SAM modified Au(1 1 1) electrode measured in 0.5 M KOH aqueous solution. The kinetic analysis indicates that SAM formation proceeded by two steps, diffusion-limited physisorption followed by chemisorption.

Electrochemical Fabrication of Nanostructured Surfaces for Enhanced Response

The objective of this work is to explore approaches to enhance electrochemical signals through sequential deposition and capping of gold particles. Gold nanoparticles are electrodeposited from KAuCl(4) solution under potentiostatic conditions on glassy carbon substrates. The number density of the nanoparticles is increased by multiple deposition steps. To prevent secondary nucleation processes, the nanoparticles are isolated after each potentiostatic deposition step by self-assembled monolayers (SAMs) of decanethiol or mercaptoethanol. The increasing number of particles during five deposition/protection rounds is monitored by assembling electroactive SAMs using a ferrocene-labeled alkanethiol. A precise estimation of the surface area of the gold nanoparticles by formation of an oxide layer on gold is difficult due to oxidation of the glassy carbon surface. As an alternative approach, the charge flow of the electroactive SAM is used for surface measurement of the gold surface area. A sixfold increase in the redox signal in comparison to a bulk gold surface is observed, and this increase in redox signal is particularly notable given that the surface area of the deposited nanoparticles is only a fraction of the bulk gold surface. After five rounds of deposition there is a gold loading of 1.94 mug cm(-2) of the deposited nanoparticles as compared to 23.68 mug cm(-2) for the bulk gold surface. Remarkably, however, the surface coverage of the ferrocene alkanethiol on the bulk material is only 10 % of that achieved on the deposited nanoparticles. This enhancement in signal of the nanoparticle-modified surface in comparison to bulk gold is thus demonstrated not to be attributable to an increase in surface area, but rather to the inherent properties of the surface atoms of the nanoparticles, which are more reactive than the surface atoms of the bulk material.

Adhesion behaviour of self-assembled alkanethiol monolayers on silver at different stages of growth

We studied the development of interfacial energy γ of alkanethiol self-assembled monolayers on Ag/mica substrates by force-displacement spectroscopy using an atomic force microscope. Self-assembled monolayers of decanethiol [CH3 (CH2)9SH] and octadecanethiol [CH3 (CH2)17SH] were studied. The growth of the monolayer as a function of time was studied by an ellipsometer for three different concentrations of decanethiol and octadecanethiol. The thiols led to well-ordered self-assembled monolayer that showed atomic resolution images by lateral force microscopy mode. The behaviour of γ showed non-monotonic variation in stages of adsorption, which was more prominent for short chain length decanethiol. The γ reached a limiting value of 12 mJ m−2 for decanethiol and 7 mJ m−2 for octadecanethiol.

Electrochemical Metal Deposition on Top of an Organic Monolayer

Electrochemical deposition of metals (platinum or gold) only on top of an organothiolate, 1,4-benzenedimethanethiol (BDMT) or hexanedithiol (HDT), self-assembled monolayer (SAM) on a Au(111) substrate was achieved by electrochemical reduction of PtCl(4)(2-) or AuCl(4)(-) ion, which was preadsorbed on one free thiol end group of the dithiol SAM formed on a Au surface, in a metal-ion-free sulfuric acid solution at potentials more negative than the reduction potential of the metal ion. Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) measurement after the reduction of preadsorbed PtCl(4)(2-) ion on BDMT/Au(111) electrode showed the presence of Pt not underneath but on top of the BDMT SAM. After a negative potential scan of the Pt/BDMT/Au(111) electrode to -1.30 V in 0.1 M KOH solution, a typical cyclic voltammogram of a clean Au(111) electrode was obtained, showing that the BDMT SAM with a Pt layer was reductively desorbed. These results proved that a Pt-BDMT SAM-Au substrate sandwich structure without a short circuit between the two metals was successfully constructed by this technique. Furthermore, a decanethiol (DT) monolayer was constructed on a Au layer, which was formed by the reduction of preadsorbed AuCl(4)(-) ion on HDT/Au(111) electrode. The formation of DT/Au/HDT/Au(111) structure was confirmed as two cathodic peaks corresponding to reductive desorption of DT from Au on top of the HDT/Au(111) at -0.97 V and that of Au/ HDT from Au(111) at -1.12 V were observed when potential was scanned negatively to -1.35 V.

Electrocatalytic oxidation of glucose at gold–silver alloy, silver and gold nanoparticles in an alkaline solution

Abstract Gold, silver and gold–silver alloy nanoparticles capped with decanethiolate monolayer shells (DT-Au, DT-Ag and DT-Au/Ag) were synthesized, with core sizes 2.3 (±1.0), 3.3 (±1.0) and 2.0 (±1.0) nm, respectively. To activate the synthesized nanoparticles for the electrocatalytic oxidation of glucose, nanoparticles were treated at 300 °C for 2 h. Heat-treated nanoparticles surfaces were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and cyclic voltammetry (CV). The elimination of C–H alkyl chains and thiolates from DT capped Au and Au/Ag nanoparticles was evident post heat-treatment by TGA, FT-IR and XPS investigations. In DT-Ag nanoparticles, C–H chains from DT were eliminated by heat-treatment, though thiolate was still present on nanoparticle surfaces. However, the thiolate from DT was eventually removed by further oxidation and reduction cycle treatments in an alkaline solution. After heat-treatment at 300 °C for 2 h, the surface content ratio of Au and Ag changed from Au:Ag(84:16) to Au:Ag(73:27). This tendency to increase the surface content ratio of Ag after heat-treatment was also observed in other Au–Ag alloy nanoparticle content ratios. Results from cyclic voltammograms at Au/Ag nanoparticles modified PFC electrodes in H 2 SO 4 and NaOH solutions indicated that the distribution of Au and Ag atoms of Au/Ag nanoparticles on nanocrystal surfaces is homogeneous. Electrocatalytic peaks for glucose oxidation in a 0.1 mol dm −3 NaOH solution were observed around −0.4 and 0.4 V (vs. Ag/AgCl) at heat-treated Au nanoparticle modified carbon electrode and around −0.4 and 0.6 V at heat-treated Au/Ag nanoparticle modified electrode, which correspond to the oxidation of glucose and further oxidation of gluconolactone generated by the first oxidation peak (−0.4 V), respectively. It is interesting to note that the catalytic current at Au/Ag nanoparticle modified electrodes was observed from ca. −0.75 V, which represents a negative potential shift of ca. 0.1 V compared to that at Au nanoparticle modified electrodes. This result indicates that Au–Ag alloy nanoparticles are effective catalysts for the electrocatalytic oxidation of glucose. At both Au and Au/Ag nanoparticles, aldose-type monosaccharides showed catalytic oxidation peaks in an alkaline solution, however ketose-type monosaccharides did not show any catalytic peaks in the potential region of −0.8 ∼ 0.8 V. After the controlled-potential electrolysis at a potential of −0.3 V, gluconolactone (or gluconate, a two-electron oxidation product) was only detected at a current efficiency of 100% at Au and Au/Ag nanoparticles modified carbon electrodes. In the case at 0.3 V, oxalate (an 18-electron oxidation product) and gluconolactone as the main product were detected at Au nanoparticle modified electrodes, and formate (a 12-electron oxidation product) in addition to oxalate and gluconolactone as the main products were detected at both Au/Ag and Ag nanoparticles modified electrodes. These results indicate that the catalytic selectivity at a potential of 0.3 V would be strongly governed by silver atoms containing Au/Ag nanoparticles surfaces.

Self-Assembled Monolayers of Alkanethiols on Gold Prepared in a Hexagonal Lyotropic Liquid Crystalline Phase of Triton X-100/Water System

In this paper, we have reported a new method of preparing self-assembled monolayers (SAMs) of decanethiol and hexadecanethiol on gold surface by using a lyotropic liquid crystalline phase as an adsorbing medium. The stability and blocking ability of these SAMs were characterized using grazing angle Fourier transform infrared (FTIR) spectroscopy and electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy. The lyotropic liquid crystalline medium possesses a hexagonal structure consisting of a nonionic surfactant Triton X-100, water, and the corresponding thiol, which provides a highly hydrophobic environment to solubilize the alkanethiols and later to facilitate their delivery to the gold surface. We find that the SAMs formed from the hexagonal liquid crystalline phase are highly compact and have excellent electrochemical blocking ability towards the redox probes compared to conventional SAMs prepared from commonly used organic solvents such as ethanol. From the impedance studies, we have determined the capacitance of the monolayer-coated electrodes and the surface coverage of the SAM, which has been found to be >99.98% on gold surface. We have also estimated the extent of ionic permeability through the film and measured the rate constants for the redox reactions on the SAM-modified electrodes. Our results show that the rate constants of [Fe(CN)6](3-/4-) and [Ru(NH3)6](2+/3+) redox couples are very much lower in the case of monolayers prepared in liquid crystalline phase compared to the SAM formed in 1 mM thiol in ethanol solution, suggesting a better blocking ability of the SAMs in the former case. From the grazing angle FTIR spectroscopic studies and capacitance measurements, we have ruled out any coadsorption of surfactant molecules on the Au surface. These results suggest that SAMs of very low defect density and extremely low ionic permeability can be obtained when a hexagonal lyotropic liquid crystalline phase is used as an adsorbing medium.

Preparation and Structure of a Low-Density, Flat-Lying Decanethiol Monolayer from the Densely Packed, Upright Monolayer on Gold

This study investigates the formation of low-density, flat-lying decanethiol chemisorbed on Au prepared by heating the surface covered with a densely packed, upright monolayer to a surface temperature above that of the onset of desorption. We determined conditions for preparing the low-density phase by observing the evolution of the photoemission spectrum as a function of the surface temperature using polarized ultraviolet light and by utilizing scanning tunneling microscopy. The preparation conditions were similar for single- and polycrystalline gold surfaces. Once the low-density decanethiol phase was formed, reflection absorption infrared spectroscopy was employed to determine the orientation of the carbon chain backbone with respect to the Au surface. The nature of the valance electronic structure for flat-lying decanethiol is described.

The use of CTAB to control the size of copper nanoparticles and the concentration of alkylthiols on their surfaces

Copper nanoparticles protected by self-assembled monolayers of decanethiol have been prepared by the controlled reduction of aqueous copper salts using the modified Brust synthesis. For comparison, the nanoparticles are synthesized on different concentration of cetyltrimethylammonium bromide (CTAB). TEM, XPS, XRD, IR have been employed to investigate the structure and formation of these nanoparticles. It was found that CTAB plays a critical role in controlling stability and ease to formation 3D SAMs during synthesis.

Spectrum of the interaction potential between He and as-deposited self-assembled monolayers of decanethiol chemisorbed on Au(111)

Abstract In this article, full monolayers of standing-up molecules of decanethiol chemisorbed on Au(1 1 1) have been grown at 275 K in ultrahigh vacuum and studied by low energy He atom scattering in the temperature range between 40 and 60 K. The specular peak intensity has been measured on the as-deposited layer and features associated to resonances with bound states of the He atom interacting with the CH3 groups of the organic molecules have been observed. Through the analysis of the angular position of these features four bound state levels have been determined. The comparison between the levels determined for the as-deposited monolayer and those determined for the annealed one shows that the spectra are sensitive to the annealing process.